In recent years, light-emitting devices, display panels, and the like which are placed on a substrate which is flexible and light-weight have been under active research and development. For example, a thin film transistor (TFT) in which a transparent conductive oxide polycrystalline thin film containing ZnO as main material is used as a channel layer has been actively developed (see Japanese Patent Application Laid-Open No. 2002-76356).
The thin film can be formed at low temperature and is transparent to visible light, so it makes possible a flexible transparent TFT to be formed on a substrate such as a plastic plate or a film.
On the other hand, a thin film formed on a film-shaped flexible substrate by a vapor phase method has been a problem that the substrate is warped by residual stress present in the thin film. As a means for solving this problem, a method by bending the substrate during the film formation process to cancel or compensate the residual internal stress is proposed (Japanese Patent Application Laid-Open No. H06-280026).